1. Field of the Invention
The present invention relates to a noise suppressor which suppresses noise transmitted through a current-carrying wire.
2. Description of the Related Art
Power electronics devices such as switching power supplies, inverters, lighting circuits for lighting fixtures include a power converter circuit which converts power. The power converter circuit includes a switching circuit which converts a direct current into an alternating current with a rectangular wave. Therefore, the power converter circuit produces a ripple voltage with a frequency equivalent to the switching frequency of the switching circuit or noise associated with the switching operation of the switching circuit. The ripple voltage or noise adversely affects other devices. Therefore, it is necessary to arrange a means for reducing the ripple voltage or noise between the power converter circuit and the other devices or lines.
Moreover, recently as a communications technology used for the establishment of a home communication network, power line communications holds promise, and the development of the power line communications has been advanced. In the power line communications, a high frequency signal is superimposed on a power line to establish communication. In the power line communications, noise is produced on the power line by the operations of various electrical and electronic devices connected to the power line, and the noise causes a decline in communication quality such as an increase in error rate. Therefore, a means for reducing noise on the power line is necessary. Moreover, in the power line communications, it is necessary to prevent a communication signal on an indoor power line from being leaked to an outdoor power line.
Arranging a line filter on a power supply line, a signal line or the like is effective in preventing the noise. As the line filter, a filter including an inductance device (inductor) and a capacitor, that is, a so-called LC filter is commonly used. The LC filter includes a T-type filter, π-type filter and so on in addition to the filter including one inductance device and one capacitor. Moreover, a typical noise filter for electromagnetic interference (EMI) prevention is a kind of the LC filter. A typical EMI filter includes a combination of discrete devices such as common mode choke coils, normal mode choke coils, X capacitors and Y capacitors.
Noise transmitted through two current-carrying wires includes normal mode (differential mode) noise which causes a potential difference between two current-carrying wires and common mode noise transmitted through two current-carrying wires at the same phase.
In U.S. Pat. No. 6,549,434, an example of the T-type filter is described. FIG. 8 shows the circuit of the T-type filter. The circuit includes first and second inductors 140 and 146 which are inserted into a first current-carrying wire 126 in series, and are electromagnetically coupled to each other. The circuit further includes a series circuit including a third inductor 142 which is connected in series, a capacitor 14 and a resistor 16 which are connected in parallel, and an end of the series circuit is connected between the first inductor 140 and the second inductor 146, and the other end thereof is connected to a second current-carrying wire 128.